The invention relates to a magnetic separator having a conveyor belt conducted over a belt drum of electrically nonconductive material for the transport of the fraction to be sorted which consists of particles of relatively good electrical conductivity, having a magnet system which can be driven in rotation inside the belt drum at a speed higher than that of the belt drum, and having a collecting container arranged behind the belt drum for the electrically conductive particles separated out.
Such a device is known from U.S. Pat. No. 3,448,857. In this known device, a magnet system arranged inside a drum rotates within the belt drum at a speed of about 1500 rpm, while the conveyor belt conveys the fraction to be sorted to the belt drum and thus to the magnet system at a speed of 1 m/sec to 1.5 m/sec. The drum in which the magnet system is arranged has an outside diameter which corresponds approximately to the inside diameter of the belt drum, and a small constant air gap is developed between magnet system and belt drum. During operation, a relative movement is produced between the conveyor belt and the drum in which the magnet system is contained, and this difference in speed has the result that the magnetic lines of force intersect the electrically conductive particles fed on the conveyor belt, whereby currents are induced the strength of which depends on the electrical conductivity of the particles. In the particles of greater electrical conductivity a stronger current is produced which has the result that these particles follow a trajectory into the collecting container arranged in front of the belt drum. Particles of lower electrical conductivity, on the other hand, remain near the conveyor belt and fall off the conveyor belt shortly before the collecting container.
Despite extensive precautionary measures, it can happen that a ferromagnetic particle comes between the conveyor belt and its belt drum. It is also possible for such a particle to come between the belt drum and the conveyor belt through a worn place in a conveyor belt. These ferromagnetic particles are held fast due to the magnetic force and rotate on the belt drum. In order not to impair the effect of the magnetic system, the belt drum must consist of a nonconductive material. As a rule, the belt drum of such a system consists of plastic. Such plastic material, however, will melt even at relatively low temperatures. A magnetizable electrically conductive particle, i.e. an iron particle, which is stuck between conveyor belt and belt drum, is heated so strongly by the alternating magnetic field of the magnet system that it begins to glow. In this condition, the particle may work its way through the plastic of the belt drum and pass into the air gap between the outer surface of the magnet system and the inner surface of the belt drum. This may then result in severe damage to the device since the rapidly rotating magnet system entrains the iron particle and, for example, slits open the circumferential wall of the belt drum. This results in the shutting down of the system and expensive repair.
The object of the invention is to develop a magnetic separator of the type described, in such a manner that damage to the belt drum as a result of particles coming between the conveyor belt and the belt drum are avoided.